This invention relates to a structural block having sound absorbing properties, and more specifically to a sound absorbing block of molded structural material of the general type described in U.S. Pat. Nos. 2,933,146 and 3,886,001, but with a cascaded series of internal cavities connected by internal slots to produce multiple sound absorption peaks at preselected frequency values.
U.S. Pat. No. 2,933,146 to Zaldastani and one of the present applicants describes the broad concept of forming structures such as load-bearing walls and ceilings of buildings with blocks made from a molded aggregate material such as concrete where the blocks have one or more internal cavities that communicate with a noise source through one or more substantially parallel-sided slots. Sound energy is dissipated principally by a Helmholtz resonance effect and a "block body" effect resulting from multiple reflections within the cavity. Some dissipation may be due to a resonant absorptive effect in the "tube" of air running from the slot to the back wall of the associated cavity. The Helmholtz resonator effect can be analogized to a spring-mass system where the mass is the entrained air in the slot and the spring is the air in the much larger volume of the cavity. As with any Helmholtz resonator, this acoustical resonator has a natural frequency f.sub.n at which the absorption of sound energy is maximized.
U.S. Pat. No. 3,506,089 to the present applicant and U.S. Pat. No. 3,837,426 describe improvements on the basic concept of the '146 patent. In these later patents, the configuration of the slot is designed to decrease impedance mismatching of the Helmholtz resonator and to raise the natural frequency above that achieved with a slot having a maximum dimension of the throat section alone. The '089 patent describes a first effort where the slot, instead of being parallel sided, has an outwardly flared configuration. The '426 patent describes another slot configuration, one that is inwardly flared. It also provides improved high frequency response, but also provides significant other advantages in both its structural strength (for a given natural frequency) and use. All of these designs shown in the '146, '089 and '426 patents use one externally communicating slot in association with one internal cavity to produce a resonator with one natural absorption peak, even though a single block may contain multiple such resonators.
U.S. Pat. No. 3,866,001 discloses yet a further improvement where a septum, usually a thin metallic sheet, is placed in the cavity. The septum exhibits a differential sound transmission, reflecting high frequency sounds within a "front" volume and transmitting lower frequencies sounds to a "rear" volume remote from the associated slot. Incident sound energy, depending on its frequency, "sees" two cavities with different volumes. This effect results in two or more absorption peaks for each cavity, depending on the number of septa used. Varying the location of the septum, or septa, within a cavity provides an ability to tune the frequency response to achieve absorption peaks at or near desired values.
While these inventions have generally proven to be commercially successful, there are nevertheless certain disadvantages associated with the use of septa. Metallic septa are themselves costly and they must be inserted manually into each cavity, thereby increasing the labor cost associated with manufacture. In some embodiments septa are bonded to fibrous filler material and inserted together in a cavity. This approach involves the material cost of the filler and its septum and still requires a separate assembly procedure to fit the septum-filler insert into the cavity.
It is therefore a principal object of this invention to provide a sound absorbing structural block that can achieve multiple resonance absorption peaks at preselected values, but does not utilize a metallic septum or an equivalent structure.
Another object is to provide a sound absorbing block with the foregoing advantage that can be formed using only conventional molding procedures for forming concrete blocks.
A further object of this invention is to provide such a block that can also absorb sound energy incident upon both its front and back walls.
Yet another object is to provide a sound absorbing structural block with the foregoing advantages that is compatible with the improvement inventions of U.S. Pat. Nos. 3,506,089; 3,837,426; and 3,866,001.
Still another object is to provide a sound absorbing structural block that is readily manufactured and has a favorable cost of manufacture as compared to prior art blocks with equivalent performance characteristics.